Abstract: We analyze the spatial trends in ice sheet mass loss in Greenland and Antarctica to determine areas where the mass loss is increasing with time versus areas where the acceleration is not significant and areas where the ice sheet is gaining mass with time using different GRACE gravity solutions and different time periods. We address the statistical significance of these results based on GRACE uncertainties and error sources. Accelerations in mass loss are examined at the ice sheet wide level and at the regional level. At the regional level, we identify the major sources of uncertainty in estimating the acceleration in mass change. To do so, we compare results using GRACE solutions that employ different de-aliasing models, we evaluate the impact of the atmospheric/ocean corrections and of the lower degree variability on the final products and we also compare results obtained using standard spherical harmonics solutions versus the regularized solution and the mascon approach applied to the harmonics. Finally, as an independent mean of evaluation of the results, we compare the GRACE acceleration data with independent observations, namely trends of ice volume acceleration from altimetry in Greenland and over key glaciers in Antarctica, and changes in surface mass balance from regional climate models and in ice velocity from interferometric SAR. We conclude on the key processes that control the estimation of the acceleration in mass loss from the GRACE data and how the results may improve in future missions, e.g. GRACE-FO.

Abstract: Understanding the mechanisms that drive the mass imbalance of the Greenland Ice Sheet (GrIS) is critical to the accurate projection of its contribution to future sea level rise. Greenland's ice mass loss has been accelerating recently, but its cause is uncertain. Using satellite Earth-gravity and regional climate model data, we show that the acceleration rate of Greenland ice mass loss from 2003 to 2012 is -12.5±1.8 Gt/yr2 results mainly from a increase of meltwater runoff (-6.3±1.3 Gt/yr2) and a decrease of precipitation (-4.6±1.3 Gt/yr2). Before the extreme surface melting in 2010 and 2012, the decrease of precipitation (-9.9±2.8 Gt/yr2) is a larger contributor than the increase of runoff (-2.6±2.4 Gt/yr2) to the ice mass loss acceleration. Furthermore, the precipitation rate is positively correlated with the North Atlantic Oscillation (NAO) during the period of 2003 to 2012. The correlation has shifted from negative to positive around the turn of the century due to the pattern change of atmospheric circulation in the Arctic. These results indicate that climate variability is playing a significant role in the recently observed Greenland ice mass loss acceleration, implying the difficulty of projecting future sea level rise based on the recent observations of GrIS.

Abstract: The High Mountain Asia (HMA) glaciers cover an area of ~120,163 km2. The status of extensive HMA glaciers has been a major public concern because they supplement 19 of the world's major river systems in Asia, acting as a water resource for over one billion people. Quantifying the HMA glaciers mass balance is still arguably elusive: recent published mass balance estimates range from -4 to -87 Gt/yr, depending on the data source (GRACE, ICESat or in situ) and different data spans. Here we present a study on GRACE derived glacier mass balance by further analyzing contributing error sources towards quantifying a more robust uncertainty. Our results show that glaciers in this region retreated at a rate of -31±7 Gt/year from January 2003 through December 2011, representing a significantly higher estimate than some contemporary studies. In addition to the limitation of relatively short data span, we find that vastly distributed lakes and wetlands within the Tibetan Plateau and their response as natural reservoirs with respect to abnormal high precipitation in certain years are the primary causes of positive mass gain signals observed by GRACE, and this effect has not been simulated, or at least not fully simulated, in current hydrology models.

Abstract: Estimates of glacier mass balance based on GRACE data are currently experiencing a large increase in popularity. Whilst assessments for the two ice sheets are slowly converging towards consensus estimates, large discrepancies are still found for the mass balance of regions outside the poles. Especially for High Mountain Asia, large differences are found between GRACE based estimates and estimates based on other approaches, such as laser altimetry or the interpolation of ground based measurements.

In this contribution, we present an ensemble-like estimate for the glacier mass balance of the Tien Shan mountain range, Central Asia. The ensemble is composed of a series of independent estimates based on GRACE data, laser altimetry data deriving from the ICESat mission, and direct glacier mass balance estimated carried out in situ. For each set of estimates, we consider a range of different techniques and approaches, in order to construct an empirical confidence interval for the presented results. We show that within the estimated uncertainty, all three methods give a consistent picture and estimate the mass balance of the region to -7.1±5.6 Gt/a during the period 2003-2009.

We attribute the agreement between the three sets of approaches to the fact that, compared to other regions within High Mountain Asia, the Tien Shan mountain range has both the largest mass-change signal and the best available data coverage.

Abstract: Land ice mass evolution is determined from a new GRACE global mascon solution. The solution is estimated directly from the reduction of the inter-satellite K-band range rate observations taking into account the full noise covariance, and formally iterating the solution. The new solution increases signal recovery while reducing the GRACE KBRR observation residuals. The mascons are estimated with 10-day and 1-arc-degree equal area sampling, applying anisotropic constraints for enhanced temporal and spatial resolution of the recovered land ice signal. The details of the solution are presented including error and resolution analysis. An Ensemble Empirical Mode Decomposition (EEMD) adaptive filter is applied to the mascon solution time series to compute timing of balance seasons and annual net balances. The details and causes of the spatial and temporal variability of the land ice regions studied are discussed.

Abstract: For over three decades, satellite laser ranging (SLR) data have recorded the global nature of long-term variations in the Earth's gravity field. A significant part of this signal is due to the readjustment of the Solid Earth in response to the mass change associated with the formation and melting of the polar ice sheets. Two decades ago, the time varying signal detectable was the composition of a linear trend and the seasonal signal in the Earth's gravity field, in particular, in the gravitational degree-2 zonal spherical harmonic J2. Analysis of the most recent time series of 30-day SLR-based estimates of Earth's dynamical oblateness, indicates that the long-term variation of J2 appears to have a quadratic component. Although the primary trend is still expected to be linear due to global isostatic adjustment (GIA), there is an evident deceleration ( ) in the rate of the decrease in J2 during the last few decades, likely due to changes in the rate of the global mass redistribution associated with the melting of the glaciers and ice sheets. This paper presents a global constraint on the ice sheet mass change since 1975 based on the secular variations in the lower degree coefficients determined from SLR data.

Abstract: Present Day Mass Transportation (PDMT) is an important geophysical phenomenon observed in the earth system. GRACE satellite mission provides unprecedented spatial and temporal data resolution to monitor the PDMT. However, GRACE observations are contaminated by signals from past glacier melting, preventing us from extracting mass transportation signal directly from observations. Despite numerous discoveries achieved so far, the results are still affected by Glacial Isostatic Adjustment (GIA) signals in both Greenland and Antarctica. One obvious problem is the lack of additional constraints in the interior of the Polar Regions to separate the coupled geophysical processes. Satellite altimetry supplies us the critical observations in the interior of Greenland and Antarctica. Earlier studies try to compare mass balance results from ICESat and GRACE results, but direct combination of those observations is difficult due to different spatial resolutions of the data. We developed a new inversion method to combine both ICESat and GRACE observations, along with other geodetic data sets. The simulated results show about 80% (15GT) improvement in PDMT uncertainty by including the altimeter data in Greenland in the presence of observation noise.

Abstract: We have examined the scale and spatial distribution of the mass change acceleration in Greenland and its statistical significance, using processed gravimetric data from the GRACE mission for the period 2002- 2011. Three different data products - the CNES/GRGS, the Dutch DMT-1b and the GGFC GRACE solutions - have been used, all revealing an accelerating mass loss in Greenland, though with significant local differences between the three data sets. Compensating for leakage effects, we obtain acceleration values of -18.6 Gt/yr2 for CNES/ GRGS, -8.8 Gt/yr2 for DMT-1b, and -14.8 Gt/yr2 for GGFC.

We find considerable mass loss acceleration in the Canadian Arctic Archipelago, some of which wil leak into the values for Greenland, depending on the approach used, and for our computations the leakage has been estimated at up to -4.7 Gt/yr2.

The length of the time series of the GRACE data makes a huge difference in establishing an acceleration of the data. For both10-day and monthly GRACE solutions, an observed acceleration on the order of 10-20 Gt/yr2 is shown to require more than 5 yrs of data to establish with statistical significance. In order to provide an independent evaluation, ICESat laser altimetry data have been smoothed to match the resolution of the GRACE solutions. This gives us an estimated upper bound for the acceleration of about -29.7 Gt/yr2 for the period 2003-2009, consistent with the acceleration values and corresponding confidence intervals found with GRACE data.

Abstract: In this study, we used the regional atmospheric climate model RACMO2 to simulate the surface mass balance (SMB) over Antarctica and Greenland. RACMO is a high-resolution regional climate model (~11 km for Greenland and ~27 km for Antarctica) forced at the lateral boundaries and the sea surface by reanalysis datasets from the European Centre for Medium-Range Weather Forecasts (ERA-40 and ERA-Interim). The monthly SMB datasets are available from 1960 to 2012 and 1979 to 2012 for Greenland and Antarctica respectively. The time period of RACMO is sufficient to estimate the natural periodic oscillations over Antarctica and Greenland, and the influence of these climate oscillations on the calculation of longterm trends. Here we estimate the natural periodic oscillations using a novel method called empirical mode decomposition (EMD). This method is empirical because the local characteristic time scales of the data itself are used to decompose the time series. In this method, higher frequency oscillations are captured in the first mode and subsequent modes have lower average frequencies. Using this method, we observe semi-annual oscillations (SAO), annual oscillations (AO), Quasi-biennial Oscillations (QBO), El Nino-Southern Oscillations (ENSO) and oscillations relating to the solar cycle over Antarctica and Greenland.